Weird Mirror Glitch?
Sounds strange indeed. Perhaps try this checklist:


Edit Mode> T> Options> X Mirror = Unchecked.
Alt+S, Alt+R, Alt+G to clear transforms. 
Ctrl+A> Location then Rotation then Scale to apply transforms.
Ctrl+S> Cursor to Center to set your 3D Cursor to the center of the world.


If none of this fixes the issue, you may also want to try exporting and re-importing a mesh format like .obj or .dae.

I hope this helps. Good luck with your project!